The invention relates to a dual ultrasonic transducer probe for use in a Doppler based ultrasound system for blood flow measurement and determination of associated hemodynamic parameters, and a blood vessel diameter determination method.
U.S. Pat. No. 4,370,985 discloses a Doppler based ultrasound probe device for measuring a blood flow rate and a blood vessel diameter. This technique is based on the continuous transmission of ultrasonic waves.
EP 0150672 discloses a process and device for determining the velocity and rate of flow of a fluid in a pipe by using a Doppler echographic method. Here, two mutually attached wave-train transmitter-receiver units are used and oriented with respect to a pipe such that the axis of one of the units is perpendicular to the axis of the pipe. According to this technique, the transit time between the transmission of a wave train by this transmitter-receiver unit and the reception of the reflected train is measured for calculating the diameter and perpendicular cross-section of the pipe.
In U.S. Pat. No. 4,103,679 to Aronson, there is illustrated and described a Doppler based ultrasound system for blood flow measurement in a blood vessel which requires that an ultrasound transducer array be so disposed relative to the blood vessel""s longitudinal axis that a Pulse Wave ultrasound beam emanating therefrom intercepts the blood vessel""s longitudinal axis at a variable beam inclination angle xcex8, whereby blood flow measurement can be quantitatively measured independent of the beam inclination angle.
An article entitled xe2x80x9cNew, Angle-independent, Low-Cost Doppler System to Measure Blood Flowxe2x80x9d by M. Skladany et al., The American Journal of Surgery, Volume 176, August 1998, pgs. 179-182, illustrates and describes a similar Doppler based ultrasound system for blood flow measurement.
Another technique based on the transmission of pulses of two ultrasound waves aimed at determining the blood velocity is disclosed in WO 97/24986. This technique is based on the zero-crossing method for frequency measurement of Doppler shifts and the use of FM modulated or pulse signals with range clipping for localizing velocity measurements.
However, the aforementioned references neither address the practical difficulties involved with ensuring that an ultrasound beam is correctly positioned with respect to a blood vessel""s longitudinal axis, nor the accurate measurement of a blood vessel""s diameter, both factors playing a major role in an accurate blood flow measurement determination.
There is accordingly a need in the art to facilitate measurements of a blood vessel diameter, as well as a blood flow and velocity profile at the blood vessel axis, by providing a novel dual ultrasonic transducer probe and a method of blood vessel diameter measurement utilizing a pair of ultrasound beams.
The main idea of the present invention consists of the following. Two transducers in the probe should be oriented with respect to each other such that ultrasound beams generated by the transducers define beam propagation axes intercepting at a certain acute angle. The transducers should be desirably positioned with respect to the blood vessel under measurements, namely such that each of the beam propagation axes intercept the longitudinal axis of the blood vessel. This can be implemented by displacing the transducers with respect to the blood vessel (either manually or by means of a specifically designed support assembly) and performing preliminary measurements of the blood vessel diameter.
According to the invented method, once the probe is desirably positioned, measurements are carried out consisting of insonating the blood vessel with two pulse-wave ultrasound beams, in a manner to substantially simultaneously (in comparison to the physiological time scale) obtain multiple sample volumes at successive coordinates (gates) all along each of the beam propagation axis. In other words, for each of the beams an amplitude vector of the reflections with Doppler shifted frequencies is obtained as an n-element vector. By applying the complex demodulation technique, which utilizes the synchronous multiplication of the input real vector of reflection amplitudes on two periodic functions with 90xc2x0-shift in phase, and a low pass filtering, the n-element vector of complex values (I and Q) for each of the beam is obtained. By this, the central frequency of the complex vector is shifted from that of the ultrasound pulse towards zero frequency. By repeating the ultrasound pulses transmission/receiving procedure m times, an nxc3x97m two-dimensional matrix Eij of reflection amplitude values is obtained for each of the beams. Here, i is the gate coordinate index (i=1, . . . , n) and j is the time coordinate index (j=1, . . . , m). It should be understood that each of the reflection amplitude values is complex and is indicative of the amplitude and the phase of the reflection at the respective gate at a certain time. By processing and analyzing these matrices (for two beams), the diameter of the blood vessel can be calculated, as well as dynamic characteristics of the blood flow, such as Doppler shifts, inclination angles, velocity, and velocity profile along the ultrasound beam.
The present invention actually enables for automatic location of the central axis of the blood vessel. Therefore, by measuring the time variations of the detected reflection at this location, and calculating blood flow velocity values, the velocity profile at the central cross section of the blood vessel along the ultrasound beam can be determined.
Thus, in accordance with a first aspect of the present invention, there is provided a dual ultrasonic transducer probe for use in a Doppler based ultrasound system for blood flow measurement, the probe comprising: a housing containing first and second ultrasound transducers each operable in transmitting and receiving modes, the transducers producing first and second ultrasound beams propagating along first and second beam propagation axes, the first and second transducers being oriented with respect to each other such that the first and second beam propagation axes intersect at a certain acute angle, and being displaceable with respect to a patient""s blood vessel to enable desired positioning of the probe such that each of the first and second beam propagation axes intersect a longitudinal axis of the blood vessel, which is determined by performing a preliminary measurement of a diameter of the blood vessel.
The housing may be of an elongated shape, the first and second transducers being mounted at a distal end of the housing. By the manual displacement of the housing with real-time analysis of the preliminary measurements, the desired positioning of the probe can be provided.
Alternatively, a specific support assembly may be used for the probe positioning. The support assembly is rotatable about the first beam propagation axis, whereby the second ultrasound transducer rotates about the first beam propagation axis. The support assembly is displaceably mounted in the housing for displacing the first and second ultrasound transducers in tandem. The arrangement is such that both the first and second beam propagation axes intercept the blood vessel""s longitudinal axis and correspondingly subtend acute beam inclination angles xcex81 and xcex82 therewith for enabling the measurement of Doppler shift frequencies along said first and second beam propagation axes. Such a probe facilitates manipulation of its ultrasonic transducers relative to a blood vessel such that both their ultrasonic beam axes intercept the blood vessel""s longitudinal axis, and subtend acute beam inclination angles therewith. Typically, such positioning is a two step process including a first step for intercepting the blood vessel""s longitudinal axis with the first ultrasonic beam; and a second step for intercepting the blood vessel""s longitudinal axis with the second ultrasonic beam by rotating it relative to the first ultrasonic beam which is maintained in its intercepting position. The present invention is particularly suitable for accurate blood flow measurement in a human subject""s carotid artery.
In accordance with a second aspect of the present invention, there is provided a blood vessel diameter determination method comprising the steps of:
(a) providing a desired positioning of first and second ultrasound transducers relative to the blood vessel to ensure that each of first and second ultrasound beam propagation axes intercepts the blood vessel""s longitudinal axis and subtends, the transducers being oriented with respect to each other such that the first and second beam propagation axes are interceptable at an acute angle, the desired positioning being provided by displacing the transducers with respect to the blood vessel and performing preliminary measurements of the blood vessel diameter;
(b) carrying out measurements to determine the blood vessel diameter by energizing the first and second ultrasound transducers to insonate the blood vessel with first and second pulsed wave ultrasound beams, respectively, and receiving an amplitude vector of reflections with Doppler shifted frequencies for each of the ultrasound beams, wherein said amplitude vector of reflections is an n-element vector formed by complex values of the amplitudes from n successive coordinates along the ultrasound beam vector representing n successive gates;
(c) repeating step (b) m times and obtaining an nxc3x97m two-dimensional matrix of the reflection amplitudes Eij, wherein i is the gate coordinate along the beam axis, i=1, . . . , n, and j is the time coordinate, j=1, . . . , m, each of the reflection amplitude values being complex and being representative of the amplitude and phase of the reflection at the respective gate at a certain time; and
(d) processing said matrix to calculate the blood vessel diameter.
The preliminary measurements ensure that the axes of the beams intercept with the longitudinal axis of the blood vessel, and include measurements of the blood vessel diameter associated with the first and second beams, respectively. The probe is displaced with respect to the blood vessel until equal and maximal values of diameters are measured for both beams. Additionally, in the center region of the blood vessel (at the location of interception between the beam and the vessel axes), the blood flow velocity values measured by the two beams have maximal and equal values.
The processing of the matrix consists of the following. A high pass filtering is applied to the matrix of the reflection amplitude values Eij along j-coordinate to remove values relating to a low frequency part of a detected signal. By this, the reflection signal associated with blood vessel walls is removed. A filtered matrix of the reflection amplitude values for each of the beams is processed to calculate an n-element real vector of time averaged amplitudes Ei along the beam propagation axis. The calculated real vector is analyzed for each of the beams to determine a beam corrected chord length L of a portion of the ultrasound beam extending between an outermost surface SO and an innermost surface SI of the blood vessel""s wall correspondingly adjacent the ultrasound transducer, and remote therefrom. The corrected chord length L and a beam inclination angle for each of the beams are utilized to calculate the blood vessel diameter D. Generally, the beam inclination could be calculated in the conventional manner, namely, from the ratio of Doppler shift frequencies in both ultrasound beams measured at the center of the blood vessel.
The blood vessel diameter determination method of the present invention can be implemented using either one of two approaches for calculating the n-element real vector of reflection amplitude values Ei along the ultrasound beam axis, and either one of two approaches for determining the portion of the ultrasound beam which intercepts the blood vessel""s wall.
The two approaches for calculating the n-element real vector Ei are based on either time or frequency domain operations on the (nxc3x97m) two dimensional matrix of reflection amplitude values Eij. The two approaches for determining the portion of the ultrasound beam which intercepts the blood vessel""s wall are based on parametric estimation for detecting initial and final parabolic like portions along the n-element vector Ei, and thresholding techniques. Due to the inclination of the ultrasound beam relative to the blood vessel, the beam corrected chord length L is preferably calculated according to the relationship: L=Pxe2x88x92B/tan xcex8, where P is a measured chord length, and B is the beam width of the ultrasound beam.
Additionally, the processing and analyzing of the matrix Eij provides for determining the inclination angles for the beams and determining the blood velocities in successive locations along the beams""axes. This allows for creating the velocity profile, and therefore calculating the blood flow rate. Considering the velocity profile across the vessel as being symmetric about the longitudinal axis of the vessel, the blood flow rate F could be calculated as:   F  =      2    ⁢          π      ·                        ∫          0          R                ⁢                              V            ⁢                          (              r              )                                ·          r          ·                      xe2x80x83                    ⁢                      ⅆ            r                              
where R is the radius of the blood vessel, r is a radial coordinate measured from the center of the vessel, and V(r) is the radial velocity dependence. If the probe is positioned correctly, then the points corresponding to the center of chords L1 and L2 are located on the axis of the blood vessel, and the coordinate of the vessel center is determined for both ultrasound beams. The velocity values measured at the center of the blood vessel in both beams should be equal after averaging the detected signals over time. This analysis could be used in addition to the equal-and-maximum diameters criteria to verify the probe positioning. The use of the vessel center coordinate is also needed to display the time dependence of velocity (or Doppler frequency) in the vessel. At the preliminary stage of the probe positioning, the Doppler signal from the center of chords L1 or L2 appears on the display to help in finding initial probe position.